1. Field of the Invention
The present invention relates to a method of manufacturing carbon nanotubes and/or fullerenes, and an apparatus for manufacturing the same.
2. Description of the Related Art
The fullerene represented by C60 discovered in 1985 and the carbon nanotube discovered in 1991 have been attracting considerable attention as a new allotrope of carbon that is different from graphite, amorphous carbon, and diamond, because they display a unique electronic material physics different from what the existing carbon material exhibits.
The fullerene represented by C60 or C70 includes great many kinds other than these, in which multiple carbon atoms are arrayed in a spherical basket to form a molecule. Further, the fullerene is easy to handle since it is soluble to an organic solvent such as benzene. In addition to the property as a superconductor or a semiconductor, it also exhibits a highly photosensitive effect, which leads to possible application as an electrophotographic sensitized material. Furthermore, a physical property that constitutes a functional material emerges by doping dissimilar elements into the fullerene, or by giving multiple chemical functional groups on the outside thereof.
On the other hand, the carbon nanotube (hereunder, simply referred to as ‘nanotube’), being a new material made up of carbon only in the same manner as the fullerene, is discovered to possess the photosensitive effect and a function as a semiconductor and the like, and is expected to be utilized in various fields of the electronics industry. Especially, since it can be made into a semiconductor or even a conductor by a slight change in the atomic arrangement (chirality), the nanotube also attracts strong expectations as an electro conductive material and/or a switching element having an infinitesimal dimension of nanometer. It also attracts attention as an electron source of the field emission type and/or a hydrogen-absorbing material, and in addition it has been tested for use as a probe for the tunnel electron microscope or a probe for the interatomic force microscope.
Conventionally, the fullerenes and the nanotubes have been manufactured by means of the laser ablation method, the chemical vapor deposition (CVD) method using an acetylene gas, and the arc discharge method using carbon rod electrodes as a raw material, and the like. The scientific grounds on which the fullerenes and the nanotubes can be manufactured by the arc discharge method, the laser ablation method, or the like are still unknown today.
With regard to production of the fullerenes and the nanotubes, various methods for mass synthesis have been examined. The resistance heating method, which was devised at the beginning, brings the tips of two graphite rods into contact in a rare gas, carries currents of some ten amperes to some hundred amperes through the rods, and thereby heats the graphite to make it evaporate. However, this method is extremely difficult to acquire a sample of few grams, and it is rarely used now.
The laser ablation method applies a pulsed laser such as a YAG laser to a graphite sample, and thereby generates plasma on the surface of the graphite to produce soot. Compared with the arc discharge method described later, this method has an advantage in producing the C60 fullerene with high efficiency, and in producing the nanotube and the fullerene with high purity.
The chemical vapor deposition method employs an acetylene gas or a methane gas as a raw material, and produces the nanotube and fullerene with high purity by means of the chemical decomposition reaction of the ingredient gas. Recently, a method of manufacturing the nanotube with high efficiency has been discovered by applying electron beams and the like to a fluorine compound with a chemical treatment applied.
It is possible to produce a high-purity nanotube, because the chemical reaction in the chemical vapor deposition method depends upon the thermal decomposition process of the raw material. Because the raw material is gas, it is also possible to continuously input the raw material. However, since chemical reaction in the chemical vapor deposition method is the thermal equilibrium reaction, the growth rate is extremely slow, which is disadvantageous. The chemical vapor deposition method using the fluorine compound as the raw material exhibits high manufacturing efficiency, and the technique thereof is effective in manufacturing a multi-walled nanotube; however, it is unfit for manufacturing a single-walled nanotube, which is promising as an electronic element.
The arc discharge method employs two graphite rods for the cathode and the anode, which are disposed in the manufacturing apparatus containing a rare gas such as a helium or argon gas, and applies some ten volts between both the electrodes to carry currents of some ten amperes. Thereby, the method generates arc discharges, resulting in raising the temperature of the anode tip up to about 4000° C., which vaporizes the anode tip to deposit soot containing nanotubes and fullerenes on the cathode and on the wall inside the apparatus. The soot contains the nanotubes and the fullerenes by some percentage. The soot containing the fullerenes is dissolved in an organic solvent such as benzene, and the fullerenes are separated and refined from the soot by the liquid chromatography method. Since the molecule size of the nanotube is quite large, there does not exist a soluble organic solvent; and the nanotubes are separated and refined from the soot by the ultrasonic method or the heat treatment method. To contain nickel compounds or iron compounds in the anode effects the catalytic action, which allows production of single-walled nanotubes efficiently.
In the foregoing arc discharge method, when the graphite rods are used for the electrodes, electrons and ions that exist in abundance in the arc plasma collide with the graphite rod of the anode. As the result, the temperature of the tip of the graphite rod rises to about 4000° C., ions and neutral particles of carbon are emitted in abundance. It is considered that the fullerenes and nanotubes are produced through the processes that deposit these ions and neutral particles on the cathode and on the inner wall of the chamber, and further deposit again on the anode. However, in the arc plasma, multifariously complicated chemical reactions are produced by collisions with excited ions and electrons, and it is difficult to stably control the quantity and kinetic energy of the carbon ions. Consequently, the method produces abundant amorphous carbon particles and graphite particles together with the fullerenes and the nanotubes, which turn into soot with these mixed. Moreover, the concentration of the nanotubes and that of the fullerenes in the soot are extremely low, which are as low as several percent.
Accordingly, to separate and refine the nanotubes and fullerenes from the soot will only produce an infinitesimal quantity of nanotubes and fullerenes. The fullerenes are soluble in an organic solvent such as benzene, which makes it possible to refine them with high purity. However, to separate and refine the nanotubes by the chemical treatment or by the ultrasonic vibration technique will not allow complete removal of the amorphous carbon and graphite particles, and the nanotubes cannot be separated with a high concentration. Thus, the conventional arc discharge method produces an extremely limited quantity of the nanotubes and the fullerenes in a single production lot, remarkably lowering the production efficiency. From these circumstances, a method of a mass production of the carbon nanotubes and the fullerenes is eagerly sought for.
In general, when the carbon nanotubes and the fullerenes are manufactured by the arc discharge method, graphite is used as the electrodes, and the electrodes generate plasma that contains the carbon group such as C and C2 by the arc discharge. This carbon group is clearly a source of the fullerenes and/or the carbon nanotubes. Therefore, in order to mass-synthesize the carbon nanotubes and the fullerenes, a mass supply of this source is expected to produce carbon materials with a quantity corresponding to the supply. However, since the supply of the source is carried out only by the electrodes themselves in the conventional technique, the anode electrode is shortened as the discharge time is extended. To mass-synthesize them will require supply of the graphite electrodes stably and continuously, and to build up such an apparatus will inevitably lead to a complicated mechanism or a large-sized apparatus.
This condition is the same as in the laser ablation method that uses graphite as the target. The laser ablation method will bring about a slightly higher content of percentage of the nanotubes and the fullerenes, but it is difficult to mass-synthesize the soot in the same manner as the arc discharge method.
Therefore, in the conventional method, the production efficiency is low and the purities of the nanotubes and the fullerenes contained in the acquired soot are low, which is disadvantageous. Especially, in order to efficiently produce the nanotubes expected as a material to achieve an electronic switching element having a dimension within some nanometers, it is required to implement an industrial manufacturing method that allows production of high-purity nanotubes in large quantities, and a manufacturing apparatus for the same. Journet C., et a)., “Large-scale production of single-walled carbon nanotubes by the electric-arc technique,” Nature, Vol. 338, p756 (Aug. 21, 1997), discloses that carbon in mass of total two grams is attained in tow minutes of synthesizing time, in a large-scale synthesis technique of single walled carbon nanotubes.